1. Field of the Invention (Technical Field)
The present invention relates to vapor processing systems for use with natural gas wells. The invention comprises a pumping system used with an engine instead of plunger lifts and can be used to remove evolved gases from hydrocarbon liquids to storage at or near atmospheric pressure.
2. Background Art
In addition to producing natural gas, many natural gas wells produce hydrocarbon liquids and water. The liquids, hydrocarbons and water, are separated from the flowing natural gas by a separator installed in the line carrying the flowing gas stream. The inline separator may operate at pressures as high as 1,500 psig or as low as 30 psig. The inline separator may separate the separated liquids into hydrocarbon and water components. The separated water is dumped to disposal, and the separated hydrocarbons are dumped to storage. The storage for the separated hydrocarbons is generally a steel tank or tanks with each tank having a capacity of 200 to 500 barrels. The storage tanks may operate at pressures as high as 16 ounces per square inch above atmospheric pressure to as low as atmospheric pressure.
An intermediate pressure separator is often used on natural gas wells that are operating at elevated pressures (150 to 1,500 psig). The intermediate pressure separator may operate at pressures of 125 to 25 psig. The intermediate pressure separator receives the total separated liquid from the inline separator. The intermediate pressure separator separates the liquid into its components, hydrocarbons and water. As described above, the water is dumped to disposal and the hydrocarbons are dumped to storage. As a result of the reduction of pressure, the intermediate pressure separator also releases most of the entrained natural gas from the separated hydrocarbons. Without a means to recover the entrained natural gas or a means designed to collect and burn the entrained natural gas, the entrained natural gas released in the intermediate pressure separator will be vented to the atmosphere and wasted. In most systems designed to collect and burn the entrained natural gas, the heat energy released by burning the natural gas is wasted to the atmosphere. A means is needed to prevent entrained natural gas from being released to the atmosphere.
Because of the reduction in pressure from the intermediate pressure separator to the storage tank, the liquid hydrocarbons dumped to the storage tanks will release additional entrained natural gas, and any component of the natural gas liquids that is not stable at the storage tank pressure and temperature will begin to evolve from the hydrocarbon liquids and change from a liquid to a gaseous state. The changing in the storage tank of hydrocarbon liquids from a liquid to a gaseous state is commonly referred to as “weathering”. Again, without a system to either recover or burn the gases released from the hydrocarbon liquids dumped to the storage tank, the gases will vent to the atmosphere and be wasted. The gases released from the storage tank are a high BTU value of approximately 3,000 BTU per cubic foot compared to the standard of 1,000 BTU per cubic foot required for residential gas. A means is needed to prevent gases released from liquid hydrocarbons from being released to the atmosphere.
For many years, systems have been made available to collect the gaseous hydrocarbons that are released from liquid hydrocarbons separated at elevated pressures and then transferred to storage tanks operating at near atmospheric pressure. In addition to operating problems that can occur with the currently available recovery systems, the biggest problem that has limited their application has been capital cost, and the systems have generally been applied to gas wells that have operated at pressures of 250 psig or less and that have produced volumes of hydrocarbon liquids in the range of 100 barrels per day or more.
Natural gas wells that can produce 100 barrels per day or more of hydrocarbon liquids do not generally require any type of artificial lift to lift the liquid hydrocarbons to the surface. In most cases, smaller volume natural gas wells do require artificial lift to lift the liquid hydrocarbons to the surface. A widely used artificial lift systems is called a “plunger lift”. The plunger is a metal device that falls to the bottom of the natural gas well tubing while the gas flow is shut off at the surface. The plunger remains at the bottom of the tubing for a period of time while the gas well builds up enough pressure to provide enough gas flow to bring to the surface the plunger and the load of liquid hydrocarbons the plunger is lifting. When the gas well is again opened, the plunger and liquid hydrocarbons rise to the surface. Often, the liquid hydrocarbons arrive at the surface as a slug that is much larger than the normal hydrocarbon liquid production of the well. The liquid hydrocarbon slug can create a volume of flash and evolved gases that will overload the vapor recovery system.
On natural gas wells where the plunger lift or other types of artificial lift creates a slugging condition that overloads the vapor recovery system, a pumping system developed by Unico, Inc. (“Unico”) can be used to lift the produced liquid hydrocarbons to the surface. Up until now, pumping of natural gas wells has been avoided because of pumping problems. Some of the problems with pumping gas wells have been gas locking (a condition where the pumping barrel fills with gas and no fluid can be pumped), gas interference (a condition where the pumping barrel only partially fills with fluid each stroke of the pump), and fluid pounding (a condition where the downward stroke of the pump contacts the fluid in a less than fluid filled barrel). The Unico pumping system presents a solution to the problems of pumping gas wells by only pumping the amount of fluids the well is producing. Pumping only the amount of fluids the well is producing prevents “pump-off” (a condition where the well bore is pumped dry thereby allowing gas to enter the pump barrel). A method is needed to eliminate gas entering the pump barrel to eliminate the problems associated with pumping natural gas wells.